The present invention relates to an image forming apparatus such as a copying machine or a laser beam printer, and more particularly to an image forming apparatus in which a transferring roller is used.
In a conventional machine such as a copying machine, there has been known a so-called transferring roller system in which an electric charge having a first polarity is given to a photoreceptor in order to form an optical image as image information projected onto the photoreceptor; from the optical image, an electrostatic latent image is formed; the electrostatic latent image is then developed by toner having a second polarity (or the first polarity in reverse development) to form a toner image; and the toner image is transferred onto a paper interposed between the photoreceptor and a transferring roller. Voltage (including an earth) having a polarity which is different from that of the toner is usually applied to the transferring roller so that the transferring roller gives so-called back charges onto the surface of the paper which does not come in contact with the photoreceptor.
In the transferring system, because excessive back charges to the paper, which are required in transfer by corona discharge are not necessary, it is possible to prevent the toner on the photoreceptor from being drawn onto the paper in the former and latter transferring steps, so that the transferring system suppresses image forming in which toner scatters around letters. The paper is interposed between the photoreceptor and the transferring roller so as to come in contact with the photoreceptor side by predetermined pressing force. Consequently, the paper is surely delivered and therefore a transfer drift, which may occur due to the slippage of the paper on the photoreceptor can be prevented.
However, when reverse development is mainly carried out to form an image having a relatively low printing ratio (which has many white portions), the paper which passed between the photoreceptor and the transferring roller might draw onto the photoreceptor side by electrostatic suction force generated by the back charges of the paper and the surface charges of the photoreceptor. This causes unstable separation of the paper from the photoreceptor.
Consequently, there has been provided an image forming apparatus including a needle for removing electric charges from the paper which is provided at the vicinity of the downstream in the paper delivery direction of the transferring roller in order to stably separate the paper from the photoreceptor (see Japanese Unexamined Patent Publication No. 269969/1989).
Because the electric charge removing needle is made of a conductive rigid body such as stainless steel, it should be displaced from the photoreceptor by a predetermined distance so as not to damage the photoreceptor. Accordingly, the electric charge removing needle cannot surely come in contact with the back of the paper, so that electric charges cannot sufficiently be removed from the paper and the paper winds around the photoreceptor. For this reason, the separation of the paper from the photoreceptor cannot be stabilized.
It is an object of the present invention to solve the technical problems above and to provide an image forming apparatus capable of stabilizing the separation of a paper from a photoreceptor.
It is another object of the present invention to solve the technical problems and to provide an image forming apparatus of which manufacturing costs can be reduced.